A prosthesis is an artifact for supplementing morphology and function of a defective portion in a body.
In dentistry, the prosthesis includes a denture, a crown, a bridge, a dental implant and the like. In surgery, it includes an artificial bone, an artificial organ, an artificial joint implant and the like. In cosmetic surgery, it includes a silicone bag, a silicone resin plate and the like. An artificial eye, an artificial ear and an artificial limb are also included in the prosthesis.
A prosthesis to be embedded in the body may be called prosthesis, and a prosthesis to be attached to the surface of the body may be called epithese.
Implants that are embedded into the body are particularly attracting attention.
For example, the dental implant is inserted into and fixed to a hole provided at the alveolar bone in a case where a tooth root of a permanent tooth has been lost due to dental caries or damage.
This dental implant is composed of a fixture (artificial tooth root) to be fixed to the alveolar bone and an abutment (support base) to be screwed to the fixture. An implant crown (artificial crown) is mounted on the abutment.
The dental implant (fixtures, abutment) is formed of metals such as titanium and titanium alloys, as well as ceramics such as alumina and zirconia. In order to avoid a risk of metal allergy, the dental implant is preferably formed of ceramics.
Since the implant is embedded in the body, sterilization (disinfection) treatment is required. The sterilization treatment for the implant may include gas sterilization with ethylene oxide gas and γ-ray sterilization treatment by γ-ray irradiation.
The sterilization with ethylene oxide gas takes 2 to 4 hours, and furthermore aeration after sterilization takes 8 to 12 hours or longer. In addition, ethylene oxide gas has high toxicity and flammability, and requires careful handling. Gas sterilization with ethylene oxide gas has extremely low work efficiency and is also difficult to handle.
On the other hand, the γ-ray sterilization treatment is superior in that the treatment time is short and there is no risk of residual gas or the like.
For the implant formed of zirconia, the γ-ray sterilization treatment has not been adopted. This is because zirconia becomes dark brown by the γ-ray sterilization treatment, and aesthetics is compromised.
However, a technique of whitening by heating a dark-browned zirconia has been developed (Patent Document 1). This technique allowed the γ-ray sterilization treatment to be adopted even when the implant was formed of zirconia.